1. Field of the Invention
The present invention relates to an electrochromic device (hereinafter referred to as ECD) and, more particularly, it relates to a transparent ECD having a short response time.
2. Description of the Prior Art
Today, display materials are enjoying widespread use in a wide range of applications, including television sets, watches, and measuring instruments. However, there has been a growing need for lightweight, low-profile, and large-area display devices due to the spread of terminal units in information processing and the technological advances in car electronics. Along with this, ECDs which have a low dependence on visual angle, a memory function and a relatively large display area have been receiving a lot of attention. However, ECDs which have become available to date have long response times and provide unsatisfactory reversibility, and therefore, have not been put into practical use.
Another promising application for ECDs is in large transparent electrochromic devices. Some liquid-crystal transparent electrochromic devices have been manufactured as test samples. However, currently available liquid crystals have drawbacks that the weatherability of polarizing films is unsatisfactory and the gap between the substrates must be kept constant. For this reason, the liquid-crystals have not been put into practical use in the dimmer devices.
A transparent ECD of a so-called oxidation-reduction complementary type combining a reduction coloring EC film and an oxidation coloring electrochromic film (hereinafter referred to as EC film) has also been proposed, as in Japanese Patent Application Laid-open Nos. 4679/1981, 114021/1983, and 139128/1983.
However, the transparent ECDs of the oxidation-reduction complementary type which have become available so far have different color tones in the two EC films. Therefore, the color quality is lowered, or a low coloring efficiency of one of the two films results in a poor additive effect of coloration.
Another ECD for coloring in hues of similar tones is also known, as disclosed in Japanese Patent Application Laid-open No. 59-155833. In accordance with the technique disclosed there, an ECD comprises an oxidation coloring substance coloring in an oxidized state and a reduction coloring substance coloring in a reduced state which are formed on the surfaces of a pair of electrodes, respectively, and an electrolyte held between the two substances, wherein the oxidation coloring substance is a double salt containing an iron hexacyanoferrate, and the reduction coloring substance is an oxide of a transition metal.
The above-mentioned prior art ECD requires lithium ion or proton as an ion species of an electrolyte for coloring/bleadching since the ECD uses, as a reduction coloring substance, a film of a transition metal oxide formed by chemical vapour deposition, such as a WO.sub.3 film and an MoO.sub.3 film. However, the lithium ion or proton significantly reduces the life of the iron hexacyanoferrate (generally called "Prussian blue"; hereinafter referred to as PB) which is used as the oxidation coloring substance on the counter electrode (for example, Japanese Patent Application Laid-open No. 98380/1983). Therefore, conventional ECDs described above did not have use life sufficient for practical applications and have been unsuitable for industrial production.